Utilizing the energy of visible light having a wavelength of from 400 to 500 nm in sunlight, plants produce oxygen gas and organic substances from water and carbon dioxide. The photosynthetic reaction in plants is multi-stage reaction, in which oxidative decomposition of water is a reaction positioned in the starting point. On the other hand, decomposition of carbon dioxide and conversion into organic substances are the final-stage reaction of photosynthetic reaction.
A latest report has clarified that, for the oxidative decomposition of water in plants, a cluster composed of four manganese atoms and one calcium atom existing in the membrane of a chloroplast acts as a catalyst that oxidatively decomposes water with utilizing sunlight energy (B. Loll, et al., Nature, 438, 1040 (2005)).
However, the cell (Photo System II) that contains the above-mentioned cluster in the chloroplast of a plant contains only four manganese atoms that bring about a catalytic effect of oxidatively decomposing water, and therefore in future, it is extremely difficult to artificially synthesize a large quantity of oxygen gas through industrial oxidative decomposition of water by utilizing the reaction of oxidative decomposition of water by plants. Heretofore, as an example of oxidative decomposition of water by an artificial catalyst substance, there is mentioned an example with a titania catalyst or a dye sensitizing catalyst. However, oxidative decomposition of water with a titania catalyst requires previous irradiation with UV rays having an energy level of from about 3.0 to 3.2 eV. The necessary energy for oxidative decomposition of water with a dye sensitizing catalyst is about 3.0 eV and is lower than that with the titania catalyst, but the dye sensitizing catalyst is a chemically unstable material and its practical use is not as yet realized. Accordingly, a catalyst material is not found as yet, which can efficiently attain oxidative decomposition of water with visible light having an energy level of from 2.0 to 2.5 eV like in actual photosynthesis by plants. In addition, it is also extremely difficult to decompose a large quantity of carbon dioxide on an industrial scale to convert it into organic substances, by utilizing the reaction of a plant that decomposes carbon dioxide in air to synthesize organic substances.
As in the above, there is known no example of efficiently generating oxygen gas through oxidative decomposition of water only with sunlight energy like in plants according to already-existing artificial catalyst technology, and in fact, neither material nor method has been found out that exhibits an excellent catalytic capability for decomposing carbon dioxide to convert it into organic substances.